For the removal of substances normally contained in the urine and for the withdrawal of fluid, different processes are used in the case of chronic kidney failure for the extracorporeal treatment and/or purification of the blood. In hemodialysis, the blood of the patient is purified outside the body in a dialyser. The dialyser is equipped with a blood chamber and a dialysis fluid chamber which are separated by a semipermeable membrane. During the treatment, the blood of the patient flows through the blood chamber. In order to effectively remove substances normally contained in the urine from the blood, fresh dialysis fluid flows continuously through the dialysis fluid chamber.
While, during hemodialysis (HD), the transportation of the small molecular substances through the membrane is determined mainly by the differences in concentration (diffusion) between the dialysis fluid and the blood, substances dissolved in the plasma water, in particular substances with a higher molecular weight, are effectively removed during hemofiltration (HF) by a high fluid stream (convection) through the membrane of the dialyser. During hemofiltration, the dialyser acts as a filter. Hemodiafiltration (HDF) is a combination of the two methods.
In hemo(dia)filtration part of the serum withdrawn through the membrane is replaced by a sterile substitution fluid which is passed to the extracorporeal blood stream either upstream of the dialyser or downstream of the dialyser. The supply of substitution fluid upstream of the dialyser is also referred to as predilution, and the supply downstream of the dialyser is also referred to as postdilution.
Devices for hemo(dia)filtration are known in the case of which the dialysis fluid is produced online from fresh water and concentrations, and the substitution fluid is produced online from the dialysis fluid.
In the known hemo(dia)filtration devices, the substitution fluid is passed to the extracorporeal blood stream from the fluid system of the machine via a substitution fluid line. In the case of predilution, the substitution fluid line leads to a connecting site on the arterial blood line upstream of the dialyser or filter, whereas during postdilution the substitution fluid line leads to a connecting site on the venous blood line downstream of the dialyser. The substitution fluid line generally exhibits a connector by means of which it can be connected either to the venous or the arterial blood line. To interrupt the fluid supply, a clamp or the like is provided on the substitution fluid line. Such a hemo(dia)filtration device is known, e.g., from EP-A-0 189 561.
Monitoring of the treatment of the blood presupposes the knowledge as to whether the substitution fluid is passed to the extracorporeal blood stream upstream or downstream of the dialyser or filter. Predilution and postdilution, for example, play a part as regards the online clearance measurement (OCM) based on a conductivity measurement since the conductivity of the dialysis fluid downstream of the dialyser depends on whether a predilution or postdilution takes place.
EP-A-1 348 458 A1 describes a process and a device for monitoring the supply of substitution fluid for an extracorporeal blood treatment device. For the detection of the supply of substitution fluid upstream or downstream of the dialyser or filter, the period of operation of the pressure waves of a substituate pump arranged in the substitution fluid line is measured. The supply of substitution fluid upstream or downstream of the dialyser or filter is recognized on the basis of the measurement of the operating time. The known process presupposes the use of a substituate pump producing pressure waves.
DE 101 15 991 C1 describes a device for detecting stenoses in a hose system. The printed document proposes to draw the conclusion of the presence of a stenosis (narrow site) in the hose system in the case of a change in the frequency spectrum of an oscillating pressure signal spreading in the hose system. The operating principle of the known device is based on the fact that the cause of the change in the dynamic behaviour of the hose system is the compliance of the hose system, i.e. the elastic yield under pressure.